Method of determining blade thickness



7, 1946 HP. MOYER 29399955@ A METHOD OF DETERMNING BLADE THICKNESS Filed Aug. 5o, 1943 Patented May 7, 1946 METHOD OF DETERMINING BLADE THICKNESS Harris P. Moyer, Toledo, Ohio, assgnor to The Aviation Corporation, New York, N. Y., a corporation of Delaware y y Application August 30, 1943, Serial No. 500,500

2 claims. (c1.` 25o-s3) This invention relates in general to an X-ray method of determining plate thickness and is more particularly described in connection with the manufacture of hollow steel propeller blades.

One of the important objects of the invention is to determine the thickness of aplate which is not easily measured by mechanical means, such as a caliper or a micrometer. Further objects of portions simultaneously in pairs to reduce error in film density; to screen the film under exposure with lead sheets to avoid secondary ray effects; to interpolate various plate thicknesses by a nlm density method; and to'check actual thicknesses with those predicted and determined by the densitometer.

Other objects will appear in the specication and will be apparent from the drawing in which '1 Fig.x1 is a somewhat diagrammatic view illustrating a'common X-ray source applied to a densitometer block and to a plate whose thickness is to be measured in accordance with this invention;

Fig. 2 is a perspective of a step penetrometer comparison plate as shown in Fig. l;

Fig. 3 and 4 are broken transverse and longitudinal sections of a film pack, portions of which are used simultaneously in Fig. 1; and

Fig. is a diagram of the readings obtained comparing density and plate thickness in inches of a plate being tested.

' While this invention is applicable to the measurement of metal thicknesses, it is particularly described as applied in the manufactureof hol low steel propeller blades, as it is of primary importance that the thickness of the metal plate be held to very close tolerances. The strength of 'the entire blade is dependent upon the strength of a blade cross section composed of a leading and trailing edge and a thrust and camber plate. VIn Amany designs of blades, the thinest section Voccurs atAv the center of the'thrust and camber -v plates, and it is therefore a decided advantage to show thelfthickness of any .particular .area in v question. .,Her'etofore', the -only methodof determining the plate thickness of 75% of the area of .as- I8 in Fig. 1 is shown in cross sections in Figs.

a finished blade was by means of the destructive method of sectioning the blade itself. By the application of the present technique of measurement by X-ray, the entire area of a iinished blade can be determined within very close limits without depending upon mechanical measuring devices.

In carrying out this invention, it is known that a given X-ray will penetrate a plate with a certain amount of intensity which is a definite function of the plate thickness. By comparing this with the intensity of X-ray penetration of test plates of known thickness, the results can be applied to similar action with a plate of unknown thickness, with the result that the un'- known thickness may then be predicted.

The thickness of the plate being X-rayed will determine the density of the negative thus produced, and this may be compared with the similar and simultaneous exposure of a portion of the same film upon a stepped test plate of know thicknesses.

Under standard conditions, the thicker the plate is, the more of the X-rays Will be absorbed in the plate and the less the X-ray negative will be exposed by the rays getting through the material to the negative.

A rectangular piece of metal 8 as shown in Fig. 2 has various steps 9, I 0, II and I2 of various thicknesses, preferably of the same Vmaterial or stock of the plate to be measured. This may bemounted upon a base plate I3 ofthe same material to be measured which constitutes a step penetrometer for the test to be made.

An X-ray-tube I5 having a suitable source of current supply is positioned above a test block penetrometer 8 and a plate I6 of which the thickness is to be measured. A lm I8 is located at a distance from tube I5 and is cut in two pieces, one being placed under the block 8, and the other underneath the plate I6. In the case of apropeller blade, lthe lm is held against the underside .of the portion to be measured by means of a bladder I9 placed below the film and inflated. A lead screen sheet 20 is placed below the film under the test block and has an upturned end 2Iwhich screens it from the plate under test. The plate under test is supported and screened from below by a lead screen sheet 22.

Asuitable type of lm, represented generally and 4 and comprises an outer cardboard holder 23 with aninside Wrapper 24 of black or opaque paper enclosing an X-ray lm'25 havingv a lead filter screen 26 at one side and a lead back filter and holding it againstthe underside oi the area to be tested, applying a portion of the same film to the underside of a stepped test block outside of the blade, screening the lm under the test block from the plate to be tested, simultaneously K exposing both portions of the film to the same X-rays under similar conditions, developing lms and obtaining lm densities for the various areas by densitometers, comparing the density of the plate area in question with the density of the steps of the test block above and below with the density of the plate area to be tested, and determining therefrom the proportionate difference between the steps and the thickness corresponding thereto, as assigned to the plate area.

HARRIS P. MOYER. 

